FIG. 1 is a sectional view showing a conventional configuration example of an electret-condenser microphone (hereinafter referred to as an ECM) 10 which lends itself to reflow soldering and is described in patent literature 1. As shown in the figure, the ECM 10 in this example consists of a vibrating diaphragm 12 and back electrode 13 contained in a conductive cylindrical case 11 and composing a condenser, and a circuit board 14 housed in an opening of the cylindrical case 11 and covering the opening.
The vibrating diaphragm 12 in this example consists of a metal film vapor-deposited on a surface of an electret film. The metal film is fixed to a conductive ring 15 and a surface of the electret is fixed to an insulating spacer 16. The back electrode 13 in this example has conductivity and is fitted in and fixed to a shoulder inside a holder 17. Also, a conductive ring 18 is fitted in the holder 17, being placed in contact with the circuit board 14 and back electrode 13.
The circuit board 14 is fixed to the ECM 10 body by folding and crimping the opening of the cylindrical case 11 inward. That is, the circuit board 14 in this example is fixed to the ring 18 by being pressed against it by the folded crimp edge (peripheral edge) 11a. Circuit elements 19 such as an FET (field effect transistor) are mounted on the inner surface of the circuit board 14 and a plurality of solder bump electrodes 21a and 21b are formed on the outer surface as electrodes for external connection. The back electrode 13 is electrically connected to electrode patterns on the circuit board 14 via the ring 18. On the other hand, the metal film of the vibrating diaphragm 12 is electrically connected to the electrode patterns on the circuit board 14 via the ring 15, cylindrical case 11, and crimp edge 11a. 
A circular pattern and an annular pattern which encloses the circular pattern are formed as the electrode patterns on the outer surface of the circuit board 14. The solder bump electrodes 21a are formed on the circular pattern to serve as signal electrodes (output terminals). Also, the solder bump electrodes 21b are formed on the annular pattern to serve as ground electrodes (earth terminals). Furthermore, the crimp edge 11a of the cylindrical case 11 is connected to the annular pattern by being held in pressing contact with it. Incidentally, the electrode patterns formed on the inner and outer surfaces of the circuit board 14 are omitted from FIG. 1.
The ECM 10 in this example is mounted on a mounting board by placing solder bump electrodes 21a and 21b of the ECM 10 on a solder layer formed on a wiring pattern on the mounting board, heating them with a reflow apparatus, and thereby melting them (reflow soldering).
Patent literature 1: Japanese Patent Application Laid-Open No. 2003-153392